Project Summary/Abstract Syphilis remains a public health threat worldwide, with an estimated 11 million new infections per year and a global burden of 36 million infections. Within the last decade there have been outbreaks of syphilis cases in major cities around the world, including within the United States, Canada, Europe, Australia and China. A particularly sharp increase has been observed in men who have sex with men, with an alarming 56% increase observed since 2011. Syphilis infections increase the risk of acquiring and transmitting HIV, and there has been an 88% increase in congenital syphilis infections, resulting from mother to child transmission in utero, within the United States since 2012. Although syphilis is curable with penicillin treatment if diagnosed early, the worldwide syphilis prevalence shows that elimination of this disease will not occur through public health control measures alone, and instead will require development of effective strategies to prevent infection with this pathogen. Development of infection prevention strategies requires an in depth knowledge of the pathogenic mechanisms used by this highly successful pathogen. The bacterium that causes syphilis, Treponema pallidum, is able to disseminate rapidly within the host during the early stages of infection to infect every organ and tissue. Minimal understanding exists surrounding the pathogenic mechanisms used by T. pallidum to undergo widespread dissemination throughout the host, and gaining understanding within this highly relevant area of study will reveal novel vaccine candidates that can be targeted to prevent establishment of infection. The long-term objective of the studies contained in this proposal is to guide effective syphilis vaccine design by increasing our understanding of the pathogenic mechanisms used by T. pallidum to cause disease. To accomplish this objective, the following specific aims are proposed: (1) to investigate the interaction of the T. pallidum adhesin Tp0751 with the host endothelial proteins laminin receptor (LamR), stomatin, and epidermal growth factor-containing fibulin like extracellular matrix protein 1 (EFEMP1), which were identified as Tp0751-binding targets during the current funding period, as well as to investigate the interaction of T. pallidum with the endothelial receptor platelet activating factor receptor (PAFr) which is hypothesized to play a role in pathogenesis due to parallels with other invasive pathogens; (2) to determine the host endothelial cell signaling pathways activated by Tp0751 and T. pallidum to facilitate treponemal dissemination; and (3) to determine the functional consequence of Tp0751/T. pallidum engagement with host endothelial cells, at both the whole cell and molecular levels. These studies will increase understanding of the critical process of T. pallidum dissemination and are expected to reveal a novel pathogenic strategy of host signaling subversion used by T. pallidum to enable crossing of cellular barriers and accessing of deeper tissues within the host.